1. Technical Field
The present disclosure relates to a shielded encapsulating structure (or package) and to a manufacturing method thereof, and in particular to an encapsulating structure for a MEMS microphone.
2. Description of the Related Art
The package of microelectromechanical systems (MEMS), such as, for example, microphones and pressure sensors, provides an electrical shielding between the region of space inside the package itself and the external environment in which the package is set. Said shielding has the function of eliminating, or at least reducing, any possible drifts in the signal of the sensor caused by interference due to electrostatic charges, for example generated by magnetic fields external to the package. Basically, said package operates according to the known principle of the Faraday cage.
In addition, the package also performs a function of mechanical protection of the sensor, albeit providing, if necessary, a certain degree of accessibility to the sensor from outside.
The electrical shielding can be obtained with different types of package.
According to an embodiment of a known type, a substrate faces an internal cavity of the package, and is insulated at the top by means of a planar cap. The internal cavity houses one or more devices, formed on the substrate. In this case, the devices housed in the internal cavity of the package are insulated from the environment external to the package by means of: the substrate; side walls, which extend starting from the substrate in a direction orthogonal to the plane in which the substrate itself lies; and the cap, coupled to the side walls in such a way as to face the substrate and the internal cavity of the package. The internal cavity thus formed is insulated from the environment external to the package. The substrate includes a ground plane, generally made of metal, for example copper. The substrate can moreover be coupled to an integrated-circuit board. Said coupling is obtained, for example, according to the standard technology of manufacturing of a substrate referred to as “ball-grid array” (BGA). In this case, conductive bumps are formed in an area corresponding to the surface of the substrate and are connected to the metal layer by means of conductive vias. Other types of substrate and/or coupling can be used. For example, as an alternative to the use of conductive bumps, it is possible to use conductive pads (or leads) coupled to one another by welding paste. Also in this case, the conductive pads are formed in an area corresponding to the surface of the substrate and connected to the metal layer by means of conductive vias.
The cap comprises a metal layer, having the function of electrical shielding between the region of space external to the package and the internal space.
The side walls are glued on the substrate using non-conductive glues or insulating adhesive tape.
To complete formation of a Faraday cage, the metal layer of the cap is electrically connected to the ground plane of the substrate by means of conductive through vias (for example, filled with resin with conductive filler material), formed on the inside of the side walls. There is thus formed a conductive path between the metal of the cap and the ground plane through the side walls, thus obtaining a Faraday cage.
FIG. 1 shows a package of the type described previously, comprising: an integrated-circuit board 1; a substrate 2, coupled to the integrated-circuit board 1 by means of a ball-grid array 4 or by means of conductive pads coupled to the integrated-circuit board 1 with welding paste; side walls 6, coupled to the substrate 2 by means of a non-conductive adhesive layer 8; and a cap 10, coupled to the side walls 6 by means of a further non-conductive adhesive layer 12. The cap 10, the side walls 6, and the substrate 2 define a cavity 14 internal to the package. In addition, the cap 10 comprises, in an area corresponding to the side directly facing the cavity 14, a metal layer 16. Formed on the inside of the side walls 6 are conductive through vias 18, for example filled with resin with conductive filler material, which are adapted to connect the metal layer 16 with a ground plane GND (illustrated schematically), via the ball-grid array 4. The conductive through vias 18 connect the ground plane GND to the metal layer 16. Generic devices and/or sensors 19 are housed in the cavity 14.
The embodiment of FIG. 1 has, however, a relatively high manufacturing cost, due to the need to form the through vias. In addition, the presence of the through vias themselves within the side walls 6 imposes a constraint on the minimum dimensions of the side walls 6, which must have a thickness sufficient to enable formation of the through vias 18, at the same time guaranteeing structural solidity of the package. For these reasons, moreover, the through vias 18 are formed at a certain distance from one another, leaving portions of the side walls 6 not electrically shielded. The Faraday cage is consequently not complete.
Further embodiments of a known type (not shown in the figure) comprise a package in which the substrate is coupled to a cap that has a recess. Said recess forms, when the cap is coupled to the substrate, the internal cavity of the package. Side walls 6 of the type shown in FIG. 1 are consequently not necessary in so far as the cap is directly coupled to the substrate. The cap comprises a metal layer formed on the inside of the recess (on the bottom and on the side walls of the recess) and in an area corresponding to the regions of coupling with the substrate. When the cap is coupled to the substrate, there is no need to form through vias of the type described with reference to FIG. 1 in so far as the walls that define the internal cavity of the package laterally are already metallized. The electrical contact between the cap and the ground plane occurs by means of the metal formed in an area corresponding to the regions that are coupled to the substrate. The substrate has, for this purpose, conductive pads for coupling with the metal of the cap. The Faraday cage that is thus formed is, consequently, complete.
This further embodiment presents, however, the disadvantage of requiring a machined cap (comprising a recess), which has a cost higher than the cost of a planar cap (for example, of the type shown in FIG. 1). In particular, the cost is typically highest in the case of packages for hermetically sealed pressure sensors, which require caps with a particular shielding of the measurement vias. In addition, due to manufacturing reasons, the depth of the recess is limited, and thus the maximum height (measured along an axis orthogonal to the plane of lie of the substrate) of the internal cavity of the package obtained is consequently limited. This embodiment is consequently suitable for MEMS sensors that require a relatively reduced height of the cavity, for example MEMS microphones in which the volume underlying the membrane of the microphone is smaller than the volume surrounding the sensor itself.